Sunday, November 23, 2008

Why does the genetic map of Europe still work?   posted by Razib @ 11/23/2008 09:58:00 AM

In the comments below Susan C asks an interesting question:
I'm still surprised that this works as well as it does, given that there were mass movements of people during the nineteenth and twentieth century.

For Europe prior to 1815, I'd expect it to work. Genealogical records show that people were very often born in the same village that their parents were, or the next village along. I would guess the rate of diffusion to be a few km per generation.

After the Napoleonic Wars, though, it goes nuts. Changing methods of agriculture (e.g. enclosure of land) meant that many rural agricultural labourers were put out of work, and had to move to the major industrial cities. This migration could easily be in the range of 100km in one generation, or even transcontinental - people emigrating to North America or Australia.

Moving forward to the Second World War, many people from central Europe fled the Nazis and came to settle in Britain.

So if you take a British person today, and ask them where their grandmother was born, likely answers range from Aberystwyth to Krakow, even if they answer "white" to an ethnicity question. (Of course there's plenty of evidence of immigration from e.g. India or the Caribbean, too)

An interesting point. Some levels of immigration and movement have always been part of European history. Think about the outflow of Huguenots after the revocation of the Edict of Nantes. The trade and migration between the Low Countries and the eastern shore of Britain. The immigration of Spaniards, Poles and Italians to France in the 19th century. The relocation of Saxons to Romania, Russia, etc.

Some thoughts:

1) Many of the immigrants, like the Huguenots, settled disproportionately in cities and towns (the Volga Russians are an exception obviously). French in Berlin, British Puritans in Amsterdam, Jewish industrial workers in East London, Asian sailors in Cardiff. And cities until recently were powerful relative population sinks. So modern European cities might be affected by past immigration (e.g., in changing the accent on dialects) culturally, but they are far less reshaped genetically than you would expect.

2) Many of the immigrants were from nearby regions. Spanish and Italian immigration to France was far higher than Polish. So the affect would be more to subtly shift the positions and centers of gravity, as opposed to rearranged the expected spatial relationship.

3) Aside from France, there wasn't much migration as a proportion of the population. The ancestors from Aberswyth and Krakow are very salient because of their exoticism. This is just subject to the same dynamics as disappearing English phenomenon.

4) They sampled from only a few locations within each nation, so the clumping is exaggerated, and combined with #3, the migration effect wasn't strong enough to change your impression. Perhaps they also generally don't sample ethnic minorities in these studies; e.g., avoiding Hungarians and Saxons in Romania.

5) Some migrations, like the expulsion of Germans from Eastern Europe after World War II, rolled back the obscuring effects of earlier movements.

I was thinking about following the notes and what not and see where the samples came from, but I'll leave it to enterprising readers. I'm sure that can answer some of these questions.